This invention relates to an electrodialytic process for recovering mixed acids from mixed salts. More particularly, the invention is directed to the recovery of mixed acids comprising HF and, for example, HNO.sub.3 from spent process materials such as pickling liquors by a process which employs a three-compartment electrodialytic water splitter.
Pickling baths, for example, are employed to remove scale, oxides, and other impurities from metal surfaces such as stainless steel. These baths comprise inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, and hydrofluoric acid, and commonly are mixtures thereof. Eventually, the acids in these baths are exhausted due to the reactions of the acids with the oxides, scale, etc. Consequently, the pickling acids are converted to a spent solution comprising acidified mixed salts. This spent solution must then be disposed of and the acids lost must be replaced. The acids, particularly hydrofluoric and nitric, are costly to replace. Moreover, the toxicity of the spent materials, especially hydrofluoric acid, can create significant environmental damage if improperly disposed of, and the large volumes of the exhausted baths add a very substantial cost to pickling processes due to the cost of disposing of these materials.
Processes for regenerating processing materials are known. For example, U.S. Pat. Nos. 3,477,815 and 3,485,581 disclose processes for removing SO.sub.2 from combustion gases employing a scrubbing solution which can be thermally regenerated, and U.S. Pat. No. 3,475,112 discloses a process for removing SO.sub.2 from combustion gases using a scrubbing solution which can be electrolytically regenerated. Recently, electrodialytic methods have been disclosed for regenerating process solutions. In U.S. Pat. Nos. 4,082,835 and 4,107,015, processes are disclosed for regenerating scrubbing solutions used in stripping SO.sub.x from flue gases by feeding the spent solutions through an electrodialytic water splitter.
While concentrated acids can be produced by electrodialytic water splitting methods, such production is limited by the current efficiency of producing these acids. For example, the current efficiency of producing 5% HNO.sub.3 is only about 0.6. Therefore, many processes employing electrodialysis produce relatively dilute acid solutions to insure high current efficiency. For example, in U.S. Pat. No. 4,504,373, a process is disclosed for regenerating a dilute sulfuric acid solution for use in the processing of rayon.